1. Field of the Invention
The present invention relates to an electrolyte composition, and more precisely, to one favorable for materials for antistatic agents, batteries and other electrochemical devices and to a method for producing the same, as well as to non-aqueous electrolyte secondary cells of high capacity and good cycle stability.
2. Description of the Related Art
An electrolyte to be used in electrochemical cells such as non-aqueous secondary cells is a medium that contains ions in accordance with its objects and has the function of transporting the ions to electrodes (this is referred to as ionic conduction). For example, in a lithium secondary cell, one typical example of non-aqueous secondary cells, the transport of lithium ions is a key characteristic.
In such cells, in general, much used is a solution-type electrolyte of high ionic conductivity, which, however, is often problematic in that, when it is sealed in cells, its solvent is exhausted or leaks out to lower the durability of the cells. Another problem with it is that, since the electrolytic solution is sealed in cells, the cells require metallic casings, and, as a result, the cell weight increases and the cell structure latitude is difficult to broaden.
To overcome the drawbacks of such solution-type electrolytes, various electrolytes of other types have been proposed these days. A gel electrolyte prepared by infiltrating a solution-type electrolyte into a polymer matrix (for example, as in R. Koksbang et al., Solid State Ionics, 69, 320, 1994) is comparable to the solution-type electrolyte as its ionic conductivity lowers little and it does not interfere with cell capabilities. However, it is still problematic in that it could not completely prevent the solvent vaporization.
Also to overcome the drawbacks of solution-type electrolytes, proposed is a solid electrolyte (polyelectrolyte). For example, it is expected that a polyelectrolyte comprising a salt dissolved in a polymer such as polyethylene oxide will solve the problems with solution-type electrolytes. However, the polyelectrolyte of this type is still problematic in that its ionic conductivity is insufficient.
One problem with most polyelectrolytes that have heretofore been proposed in the art is that their ability to transport ions is generally poor. The ion transport is one important property of electrolytes. For example, in lithium secondary cells comprising a polyelectrolyte, the lithium ion transport by the polyelectrolyte is low. Therefore, the lithium secondary cells of this type are problematic in that the charge-discharge current through them decreases in time and the cell capacity is low. For these reasons, the cells could not be built in all-purpose devices.
On the other hand, imidazolium salts and pyridinium salts having a counter anion of BF4−, (CF3SO2)2N− or the like are room-temperature fusible salts that are liquid at room temperature, and they are proposed for electrolytes for lithium ion cells. Though their ionic conductivity is high, however, they still have some problems. One is that their ability to transport lithium ions in cells is often low. In this connection, the ion transport is one important property of electrolyte as so mentioned in the above. Another is that, since they are liquid, they will leak out of cells.